The invention relates to a foreproduct for the preparation of bone cement, which contains, in addition to a gentamicin compound, either copolymers of methylmethacrylate and methylacrylate in powder form or monomeric methylmethacrylate, as its principal component.
Bone cements are used, for example, for cementing implants in place, for the anchoring of endoprostheses of the joints, in the treatment of skull defects, and for the performance of spinal fusion. They are obtained by mixing together foreproducts composed of homopolymers or copolymers of methylmethacrylate in powder form and of suitable liquid monomers, usually methylmethacrylate, a catalyst system and, in some cases, X-ray contrast agents such as zirconium dioxide or barium sulfate, for example, dyes for the identification of the cement in the body, and other additives, to prepare a plastic dough, which is then placed in the body and set "in situ" by polymerization of the monomer. The catalyst system is a so-called "redox" system composed of an organic peroxy compound, usually dibenzoyl peroxide, as the catalyst, plus a reducing component (accelerator) such as dimethyl-p-toluidine. Thus, for example, the bone cement known from German "Auslegeschrift" No. 2,229,702 is prepared from polymethylmethacrylate and a monomer mixture of methylmethacrylate and methacrylic acid esters of higher alcohols, and a catalyst system consisting of dibenzoyl peroxide and dimethyl-p-toluidine.
For prophylaxis against infections at the boundary surface between the bone cement and the bone, such as can occur when the natural hip joint is replaced with an artificial one and the socket and the femoral shank are anchored by means of bone cement, antibiotics are added to the bone cement. Such a bone cement or glue is known from German "Offenlegungsschrift" No. 2,022,117, in which penicillin, gentamicin and tetracycline are given as examples of suitable antibiotics. One product which is commercially available for the preparation of bone cement contains gentamicin sulfate.
The antibiotic is at first released from the hardened bone cement in a relatively high concentration to assure its bactericidal and bacteriostatic action. Then, however, a diminution of the concentration takes place; the release rate, which is now lower, remains relatively constant over a longer period of time, and can properly be termed a sustained release; however, the active concentration of the antibiotic is low. In this manner, infections can be largely prevented by the released antibiotic during the early post-operative hours, but due to the reduction of the concentration later infections cannot be reliably prevented or combatted.
The possibility of increasing the effective concentration by the addition of greater amounts of antibiotic is limited, because such addition would impair the mechanical strength of the hardened bone cement.